''The only time I see my mother is when I'm in the office,'' Mr. Sciubba said. Both work at Sherlock Holmes Realty in Sea Cliff on Long Island; he is the rental manager and she is a real estate agent.

It is hardly news that 20- and 30-somethings are living with their parents, camped in a back cottage or a childhood bedroom. The arrangement passes for entertainment, from the Fox sitcom ''Free Ride,'' about a college graduate who moves back in with his parents, to the comedy ''Failure to Launch,'' starring Matthew McConaughey and Sarah Jessica Parker, about a 35-year-old man who never left home.

But the cost of real estate is making plenty of strange housefellows, with siblings, elderly parents and children, work mates and friends doubling up to save on the mortgage or rent.

Some local governments are cracking down on cohabitants who are not related. In extreme cases, where day laborers and other immigrants are crowded into houses meant for single families, they are evicting.

Even the classic ''mother-daughter'' arrangement -- a single-family home with an apartment added for a parent -- is increasingly likely to be illegal, as the cost of building such an apartment to conform to building and zoning codes can run up to $100,000.

And yet people all over the suburbs are making shared housing work, with some finding happiness and comfort in living with those they know and maybe even love.

''It doesn't feel that crowded since they're all family,'' said Ismama Joseph, 52, who lives in a four-bedroom duplex in Norwalk, Conn., with her husband, two adult daughters, their husbands and the five grandchildren she cares for while the others are at work. ''But it would be impossible with strangers.''

In Fairfield County, Conn., many older people -- ''people who have lived their entire lives in Norwalk'' -- are selling and moving in with their children, according to Carol Ann Falasca, a broker at Coldwell Banker Real Estate in Norwalk. ''Their children went to the schools here and all of a sudden they're retired and they cannot afford to stay because of the taxes,'' she said.

In New Jersey, Gordon Crawford, a real estate agent with Re/Max Properties Unlimited in Morristown, said about 10 percent of his buyers have been groups of family members buying together.

One such group included Suzanne Rosengold, a single mother, who was living last year with her daughter, Hanna, in a one-bedroom rental apartment in Parsippany in the same complex as her parents.

When Ms. Rosengold's grandfather died last year, the family inherited a Florida condominium that they sold, planning to use the proceeds for a down payment on a house.

After looking at dozens of houses, the Rosengolds found a four-bedroom ranch in Lake Hiawatha, a suburb of Parsippany. All three generations moved in last July -- the first time any of them had owned a home.

The mortgage payment is the same as the combined rents the Rosengolds were paying before. ''It has really changed our whole lives,'' Ms. Rosengold said. ''My daughter has her own room now. Before, she had a converted dining room.''

Her father, Edward Rosengold, 63, said: ''I do things now that I've never done before. I have my own workshop. I build things, I repair things.''

In another three-generation household, in Fairfield, Conn., Christina and Raul Diaz live in a three-bedroom 1,750-square-foot ranch with Mr. Diaz's 59-year-old mother, Leonor Martinez. Ms. Martinez takes care of the couple's 2-year-old daughter, while they work in nearby Stamford.

But the household will be growing, Ms. Diaz said. Her mother, Beth Bodnar, 70, who recently retired, is moving in soon, and the couple hope to have another baby next year.

Ms. Diaz has been looking for about four months for a bigger house that the family can afford in Fairfield. They need about 2,500 square feet and, ideally, a ground-floor master bedroom, she said.

But pickings are slim.

''We're probably going to have to put it off another year to save more,'' Ms. Diaz said.

The couple are assuming that the real estate market is not going to change at all, she added. ''My big concern is how much equity we can get out of our existing house, because I feel like it's a buyer's market, not a seller's market,'' she said.

Baby boomers reaching retirement age who want to move to a warmer climate are also increasingly helping their adult children expand their homes or buy a larger house.

They don't want to lose total contact with their lives here after moving to Florida or Arizona, said Elayne Jassey, a real estate agent with Prudential Connecticut Realty in Stamford.

Expanding one's living space to accommodate a relative may be a good investment, in spite of the cost of complying with zoning codes and the possibility of higher property taxes.

While ''real estate prices have exploded,'' said Robert Campbell, professor of real estate finance at Hofstra University in Hempstead, N.Y., construction costs have risen more slowly with inflation. Under those conditions, he said, ''building onto your own house is one of the best things that you can possibly do.''

But where young people are concerned, multigenerational living is often seen as temporary. Matt Wolff, 31, has been living at his parents' home in White Plains, N.Y., for two years since friends with whom he shared an apartment in Bronxville, N.Y., went their separate ways.

Mr. Wolff is saving for a down payment on a co-op, ''the last bastion of affordability'' in the area, according to his father, Nick Wolff, who owns Century 21 Wolff, a six-office real estate agency based in White Plains.

''It's not a problem for us,'' the senior Mr. Wolff said of his son's situation, ''but from his standpoint, he's a grown man and wants to be on his own and have privacy.''

On Long Island, Sean Sciubba also says he won't stay in his mother's cottage forever. ''I work in real estate,'' he said. ''I've done the research. Hopefully, I'll be able to get out of there soon.''

Photos: Three generations of Rosengolds are living in a four-bedroom ranch in a suburb of Parsippany. From top left: Lana, Suzanne, Hanna and Edward. (Photo by Nancy Wegard for The New York Times); Christina and Raul Diaz of Fairfield, Conn., seek a home to accommodate both of their mothers. (Photo by Thomas McDonald for The New York Times)